摘要:

Assuming that the origin of the nonlinear interaction lies in the second‐order optical susceptibility arising from the nonlinear induced current density and using the coupled‐mode theory, the parametric dispersion and amplification of acoustohelicon waves is analytically investigated in a longitudinally magnetized piezoelectric semiconductor of noncentrosymmetric nature. The relevant experiments have not been reported. The threshold value of the pump electric fieldE0thand its corresponding excitation intensity is obtained. The longitudinal magnetic field decreases the required magnitude ofE0thfor the excitation of parametric amplification. The phenomenon of self‐defocusing of the signal in the prevailing case is found to be a consequence of the negative dispersive characteristics exhibited by the acoustohelicon waves. Numerical analyses are performed for an InSb crystal at 77 K, duly irradiated by frequency‐doubled pulsed 10.6‐&mgr;m CO2lasers. The parametric gain constant is observed to be maximum when the cyclotron frequency &ohgr;cattains the magnitude equal to that of &ohgr;0, the incident laser frequency (=1.78×1014s−1).

ISSN:0021-8979    

DOI:10.1063/1.347657

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A population inversion density on the Ti 551.4‐nm (3D01−3F2) transition in a N2‐laser‐pumped Ti‐vapor laser has been estimated from measurements of an optical gain on the laser transition, length of the laser medium, and temperature in the vapor to determine the linewidth. Ti vapor is generated by irradiation of a pulsed YAG laser light of 1 J/pulse energy and then the Ti atoms are optically pumped by a N2laser of 0.2 mJ/cm2intensity. It has been experimentally observed that, in pure Ti vapor, the population of Ti (3d24s2 3F2) is about 8×1013/cm3, the temperature in the vapor is about 9000 K, the population inversion on the 551.4‐nm transition is about 1.5×1011/cm3, and the absorption cross section of the N2‐laser light at 337.044 nm by a Ti atom is 3.0×10−15cm2.

ISSN:0021-8979    

DOI:10.1063/1.347658

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Basic experimental results obtained with a GaAs/AlGaAs double‐heterojunction bipolar transistor (DHBT) waveguide structure carrier‐injected optical intensity modulator/switch are fully described. Typical common‐emitter dc current gain obtained with a 0.25‐&mgr;m‐thick base/waveguide was ∼40, and the switching times have been determined as 1.5 ns. Optical on/off modulation ratios of up to 2.1:1 have been demonstrated at an input base current of as small as 3 mA for a device with emitter/waveguide width of 7 &mgr;m and length of 190 &mgr;m. Design and analysis of the optical switching characteristics expected for a reflection‐type X‐crossing optical switch of DHBT waveguide structure are also presented on the basis of these experimental results.

ISSN:0021-8979    

DOI:10.1063/1.347659

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A high‐power chemical oxygen iodine laser system has been developed with industrial applications in mind. The present system shows fairly high efficiency, good stability, and reasonably good beam quality applicable to practical applications. Highly efficient power transmission through optical fibers has been demonstrated.

ISSN:0021-8979    

DOI:10.1063/1.347660

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

MeV4He+implantation into single crystals of KNbO3has been used to fabricate planar optical waveguides by generating a buried layer of lowered refractive index. We characterize the optical loss of these waveguides as a function of the ion doseD, and find a minimum loss of (1.0±0.2) cm−1after implantation of 2 MeV ions at a dose of 5×1014cm−2. This is by far the lowest dose ever for which efficient waveguiding has been reported. For good guides, the observed losses are mainly due to tunneling through the refractive index barrier with nontunneling contributions of less than 30%. The reduction &Dgr;nbof the refractive indexnbin the region of the nuclear damage peak is proportional to the dose and we find &Dgr;nb/D=(7.8±1.6)×10−17cm2.

ISSN:0021-8979    

DOI:10.1063/1.347661

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A novel multistage numerical modeling approach to simulate acoustic‐wave generation and propagation of an electromagnetic acoustic transducer (EMAT) is presented. The model assumes a uniform static magnetic field in a conducting half‐space. A meander coil winding situated above the half‐space is driven by a transient high‐frequency current pulse. The numerically computed eddy currents in the conducting specimen are combined with the static magnetic field to yield spatially and temporally diffusive Lorentz forces which in turn are coupled into the elastic‐wave equation. This hyperbolic elastodynamic system is solved for the displacement field vector which gives rise to propagating elastic waves. Numerical transduction results are discussed for a generic two‐wire transient EMAT configuration and for an isotropic half‐space with electric and acoustic material parameters equivalent to those of aluminum.

ISSN:0021-8979    

DOI:10.1063/1.347662

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Measurements of the ultrasonic attenuation and relative velocity changes of longitudinal waves of frequencies between 5 and 25 MHz are reported for clear yellow Dominican amber at low temperatures. Between 4 and 130 K the attenuation and velocity change are glasslike in their temperature and frequency dependencies. At higher temperatures both properties seemed to be affected by the &agr; or &bgr; viscoelastic relaxations. Similar ultrasonic behavior is reported for commercial polymethylmethacrylate. Our results at low temperatures compare well quantitatively with data of other organic and inorganic glasses, except for the stronger logarithmic frequency‐dependent velocity change which is observed for both materials.

ISSN:0021-8979    

DOI:10.1063/1.347664

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

An ultrasonic imaging system is described by using a leaky surface acoustic wave (SAW) transducer composed of a piezoelectric ceramic thin plate/interdigital transducer/fused quartz. It is shown, both numerically and experimentally, that three operation modes of the transducer at a water‐solid interface are available. Each of the three modes has a different operation frequency range, corresponding to the leaky SAW velocity. The bulk wave in water as an acoustical coupler between the transducer and a tested object has a frequency‐dependent angle relation; therefore, the acoustic beam steering is easily controlled and flexible in design. The reflection type ofC‐mode ultrasonic imaging results are demonstrated and are promising for nondestructive testing.

ISSN:0021-8979    

DOI:10.1063/1.347745

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Multiple steady states have been observed in a 13.56 MHz chlorine glow discharge in a parallel plate radial flow reactor. Measurements of the flux and energy distribution of positive ions bombarding the electrode are reported along with radial profiles of atomic chlorine (Cl) concentration and etch rate of polysilicon for different states of the plasma. At the same operating conditions the plasma may exist in one of two states. In one state the plasma appeared radially uniform, resulting in relatively uniform radial Cl concentration profiles and etch rate. In the other state the plasma appeared radially nonuniform with a higher intensity central region (‘‘plasmoid’’). The etch rate and Cl concentration profiles were nonuniform in the presence of the plasmoid. The plasmoid generally appeared under conditions of high power and pressure and could be eliminated by adding noble gases and/or decreasing the electrode spacing. The multiple states may be a result of the strongly electronegative nature of the chlorine discharge.

ISSN:0021-8979    

DOI:10.1063/1.347746

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The ‘‘flux‐tube’’ model originated by Morozov is a very simple and numerically efficient method for simulating ion motion in plasma filters. In order to test its utility as a design tool, we compare the predictions of the model to recent experimental measurements of plasma flow in both straight and curved solenoids.

ISSN:0021-8979    

DOI:10.1063/1.347747

出版商:AIP    

年代:1991

数据来源: AIP